Why scientists root for biological pest control

The delta trap is a huge relief to tomato farmers in Nakuru

In Summary

• The efficacy of the trap is around 80-90 per cent, if the farmer instals it early

• Use of this means minimises the need for chemical pesticide application

Kenya Biologics Ltd scientist Velma Wekesa with the triangular delta trap in Josiah Ngumi’s tomato farm in Subukia, Nakuru county
Kenya Biologics Ltd scientist Velma Wekesa with the triangular delta trap in Josiah Ngumi’s tomato farm in Subukia, Nakuru county

Tomato farmers in Subukia, Nakuru county, have been confronting the menace of a pest that has mostly been chewing into the leafage of the crop. The resultant effect has been a stare at the possibility of a drop in the final product.

The pest, Tuta absoluta, which crept into the country around 2016, has previously been exclusively tackled by chemical pesticides. But now, a shift in this trend is being sought.

This is through an Integrated Pest Management approach that hugely incorporates biological control coupled with minimal chemical pesticide application.

Josiah Ngumi has been a farmer for about 10 years. He’s been farming tomatoes on a two-acre piece of land. He also grows red cabbage, spinach, capsicum and other vegetables.

While still in his childhood, he inherited from his parents a liking for farming. He’s now ventured into agribusiness.   

“The major challenge for tomatoes has been Tuta absoluta," he says. "This is a very destructive pest. It can wipe out all your harvest. But now we have found a solution to the problem.”

Dr Catherine Lengewa, partnerships director at the Centre for Behaviour Change and Communication, says, “Tomatoes are considered lucrative crops to grow since they mature fast and have a good return once harvested and sold.”

She adds, “Despite these advantages, tomatoes are known to be vulnerable to pests and diseases, including early and late blight and Tuta absoluta, making it a high-risk crop to grow and consequently prone to heavy pesticide use during its production.”

“A Social and Behaviour Change Pesticides Risk Reductions strategy is being implemented aimed at influencing a range of behaviours that promote the rational use of pesticide across the tomato value chain,” Dr Lengewa says.

The target audiences include farmers, agro-dealers, community members, market traders, extension service providers and policymakers.

The major challenge for tomatoes has been Tuta absoluta. This is a very destructive pest. It can wipe out all your harvest. But now we have found a solution to the problem
Josiah Ngumi


Duncan Chacha, a crop protection scientist at the Centre for Agriculture and Bioscience International (CABI),  says crop protection activities entail the diagnosis of pests and diseases to identify the causal organisms and effect appropriate management methods.

“One of the ways we use to manage Tuta absoluta is by trapping," he says. “One trap type is delta, which has a sticky surface put into it. There’s a pheromone to attract the adults into the trap.”

Another scientist, Velma Wekesa from Kenya Biologics Ltd, describes the delta trap thus: “It consists of a housing that has a triangular cross-section. The sex pheromone attracts the male Tuta moth, which comes into the delta housing and gets stuck on the sticky surface.”

Soon after sticking on the glue surface, they die. “So the population of the Tuta absoluta in the field is reduced and, therefore, the farmer can harvest more crop,” Chacha says.

Through this way, the chances of the males mating with the females will have been reduced and, therefore, the generational cycle of this destructive moth to the tomato fruit would be cut.

Ngumi has set two types of traps on his farm because “the pesticides we’ve been using haven’t been effective”. 

He has two deltas and three water traps. He recalls that he was once invited to a seminar by CABI through the Ministry of Agriculture, and after that came in contact with Koppert Biological Systems. These are the organisations that introduced him to biopesticides, including the snares for the Tuta Absoluta.

The uptake of the technology has been increasing over the years across the horticulture industry. Wekesa recommends the installation of the traps once the tomato crop is two-three weeks old.

When the farmer notices five to 10 insects trapped, then he would know that infestation has begun. “The next step is to increase the number of traps in the farm area.”

“The efficacy of the trap is around 80-90 per cent, only if the farmer has installed the trap early enough,” Wekesa says. “The farmer should not wait until he sees damages to the crop. It will not help.”

The 10-20 per cent shortfall will present the farmer with the option of spraying. “But at this point, he’ll use less of the chemicals and that is what we are advocating: integrated pest management, which entails combining both biological and chemical ways of controlling pests in the farm,” Wekesa says.


The moth that attacks tomatoes is active from the evening hours, from around 5 or 6 pm. In open fields, due to environmental factors, reaching 100 percent efficiency would be unrealistic, unlike a greenhouse, which is a controlled environment.

The water traps on Ngumi’s farm consist of shallow-depth circular containers filled with ordinary water, with a miniature tower-like structure housing a pheromone. He’s installed three such traps at different positions. The pest of interest remains the male tuta. “These are attracted and fall into the water and drown,” Chacha says.

CABI runs the Plantwiseplus programme, which employs several digital tools to educate farmers about the positives of using biocontrol methods. The bioprotection portal is one of the tools, among others, where farmers can get information about the pests and diseases affecting their crops, the biocontrol products in the country, how to use them, and where they can be found.

Chacha says generally, farmers rush to use pesticides to control the infestation of their crops. Consideration of the harmful effects of these pesticides on the farmers and the environment rarely features in the farmers’ minds.

In biological control, the emphasis is on minimising chemical use, not abolishing it. Some of the pests may escape the dragnets. “We occasionally spray,” Ngumi says.

Chacha adds, “When we use these pheromone traps, it means a farmer will use less of the pesticide.”  

Miriam Wanjiru, an agribusiness officer in the Agriculture department in Subukia subcounty, says, “When I visit a farmer, I tell them to do scouting. This is investigating if the plants are healthy. This should be done every morning. I recommend that they try the biological control methods to control pests or diseases.”

“My harvests have improved by over 45 per cent. I’ll probably get 200 crates,” Ngumi says. Tomato prices are not constant. They vary between Sh1,500-2,500 per crate.

He has employed five people on his farm. They weed, irrigate and stick the tomatoes to hoist them above the ground, lest they rest on a wet surface that would damage the crop. The water Ngumi uses for irrigation is pumped from a nearby stream.

Raising the tomatoes above the ground also contributes to making them clean and, therefore, more likely to attract customers. 

Please worry about the consumer,” Wanjiru says. “Let’s reduce the use of chemicals and accept that biological control also works
Miriam Wanjiru


"We don't have a specific market for our tomatoes," Ngumi says. "If you find yourself having taken your product to the market with so many farmers who’ve done so on the same day, you may find yourself stranded.” 

Farmers in the Subukia region are slowly accepting the use of biocontrol products to keep diseases and pests in check.

Fred Kinyanjui has an extensive agricultural background. He’s a former agricultural officer. He’s also previously served as a crop officer in horticulture and field crops, as an environment officer and even as a plant doctor. He’s now the proprietor of Subukia Farm Supplies.

His business doesn’t bar him to delve into agricultural extension work. “I could improve the farmers in one way or the other, especially in the use of pesticides, because this is what I have done most,” he says.

Regarding Tuta absoluta, he says, “The effective insecticide is Relay. It is used in small quantities, four millimetres per 20 litres of water. Because of the low dose, it is very effective and safe for the environment.”

"Farmers are not responding that well to the use of biological methods. They'll argue that the method does not kill the pest or eliminate disease," Wanjiru says. "So they tend to go behind your back and use chemicals."

Kinyanjui concurs with what Wanjiru says. “I have been using the traps on my farm to become a good example to other farmers,” he says.

Ngumi foresees himself in the future venturing into other areas of agribusiness. "We are talking about food security and safe food, for that matter. If I get adequate capital I would like to farm in hectares."

Wanjiru is concerned about the chemical residue that settles on tomatoes if sprayed two or three weeks before harvesting. "This is not safe for consumers.”

“Please worry about the consumer,” Wanjiru says. “Let’s reduce the use of chemicals and accept that biological control also works.”

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